This invention concerns a novel alkali-resistant glass fiber having silica, magnesia and calcia as its major components, and its preparation from readily available minerals.
Alkali-resistant glass fibers are finding increased acceptance, particularly in cementitious products where their inclusion improves the mechanical properties of such products. These improved properties permit thinner, lighter cement structures to be used as, for example, prefabricated decorative panels in cladding the outside of buildings.
Ordinary glass fibers cannot be used in cement, since such a highly alkaline environment tends to attack and deteriorate the glass. To overcome this drawback, new glass compositions with high alkali resistance have been developed. Present commercial alkali-resistant glass fibers are drawn primarily from compositions containing predominately silica but with zirconia at levels often as high as 25 weight percent as the key component. While the high zirconia content allows the glass to be continuously fiberized to small diameters and yet remain alkali-resistant, such glasses containing this expensive component require the use of high melting and fiberizing temperatures.
It is therefore a primary objective of the present invention to provide a glass composition derived from relatively inexpensive mineral sources which is highly alkali-resistant but which can be readily drawn into continuous monofilaments at reasonable operating temperatures.
Prior attempts at providing such an alkali-resistant glass fiber include that disclosed in Published European patent application No. 76,677 in which a fiber is formed from a composition containing from 20 to 30 weight percent CaO, 15 to 20 percent MgO, and the balance SiO.sub.2 using diopside alone or in combination with up to 10 percent sandstone or quartz or up to 20 percent dolomite. In Published International (PCT) patent application No. WO 84/01365, alkali-resistant glass compositions purportedly suitable for use as glass fibers are prepared from naturally-occurring pre-reacted zeolite admixed with a significant quantity of alkaline earth metal-containing material such as calcium carbonate. Japanese Kokai No. 13819/76 discloses an alkali-resistant composition for glass fiber formation consisting of 42 to 66 weight percent silica, 5 to 30 percent magnesia and 5 to 50 percent calcia. Other glass compositions for use in preparing alkali-resistant fiber which contain a major portion of alkaline earth oxide include those disclosed in U.S. Pat. Nos. 3,854,986 and 3,904,423, British Pat. No. 1,227,355, West German Offen. No. 2,361,195, Japanese Kokai No. 107309/76, 3367/80, and 166359/82, and by Kokubu et al, Kagoshima-ken Kogyo Shikenjo Nempo, 21, 21-3 and 24-7 (1974). Despite these approaches, however, the need still exists for an improved, simplified composition for producing the desired fiber.